Fix a simple compilation problem (non conforming C++).

On linux (and this is probably required by the C++ standard), #include is required to be able to use std::function. Otherwise, we get a compilation error.

applications/platform/tracker/include/ParallelExecutor.h

@@ -4,6 +4,7 @@
 #include <list>
 #include <deque>
 #include <vector>
+#include <functional>
 
 #include <mutex>
 #include <condition_variable>